Hi everyone

While I have mastered almost all easy-puzzles without trouble, I am completely stuck at this one:

They given example just doesn’t tell me how this cost thing works, if there are multiple numbers. Well at first I thought I would understand:

numbers: 1,2,3

1+2= 3

3+3= 6

Total cost: 9 (correct)

numbers: 1,3,4,4

1+3= 4

4+4= 8

8+4= 12

Total cost: 24 (correct)

numbers: 9,9,9,9,9,9,9,9,9 (9x9)

9+9= 18

18+9= 27

…

72+9= 81

Total cost: 396 (WRONG - correct answer: 261)

The only way I get to 261 is, when I do the following:

9+9= 18

18+9+9= 36

36+9+9= 54

54+9+9= 72

72+9= 81

Total cost: 261

But this algorithm doesn’t work with the first two examples. I cannot find an algorithm that works for all of the 3 examples.

Can anyone help me to understand this puzzle? I have already spent at least 5 hours trying to understand what is wanted… I would really appreciate if someone could bring some sense into this